Alloys of noble metals with augmented quality factors of surface plasmons

ABSTRACT

A set of improved substrates for plasmonic applications characterized by specific characteristics of surface plasmons, such as resonance frequencies, manifesting, notably, as a change of perceived color of said alloys, as well as alteration of surface plasmon energies and generation efficiency compared to the constituent elements. The disclosed compositions include alloys of gold, platinum, and palladium with other chemical elements, especially alkali metals (such as potassium, and rubidium), alkali earth metals (such as magnesium and barium) as well as transition metals (such as tin and zirconium).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/624,097 filed on Jan. 30, 2018 to which priority is claimed under 35U.S.C. 119 and the contents of which are hereby expressly incorporatedby reference herein.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to alloys of precious metals such as gold,platinum, and palladium with other chemical elements.

BACKGROUND

Phenomena associated with energy and information transfer as well astransient or permanent chemical transformations that occur at theinterface between materials with different electromagnetic propertiesare commonly referred to as plasmonic phenomena. These are produced as aresult of generation of coherent delocalized electron oscillationsgenerally known as surface plasmons. Such surface plasmons may befurther differentiated as surface plasmon polaritons, and localizedsurface plasmons; however, for the purposes of the disclosed inventionthe difference is insubstantial and both surface plasmon polaritons andlocalized surface plasmons are further referred to generically assurface plasmons.

Surface plasmon-related phenomena are utilized in a number of industrialand research applications as diverse as high speed optical switching,formation of optical circuits, optical cloaking, superlensing,surface-enhanced spectroscopy, solar light capture, directtransformation of solar light energy into electric energy,photocatalytic chemical processes, and plasmon-based electronics.

Performance of plasmonic systems depends on the fundamental propertiesof the underlying material (hereinafter termed ‘substrate’), as both theelectromagnetic surface effects and chemical surface effects areaffected by some or all of the following factors:

composition of the substrate (e.g., pure chemical elements oralloys/mixtures, and if such, of which constituent chemical elements;chemical compounds and, if such, of which constituent chemical elementsand in what ratios, structural arrangements and types of chemical bond);

fine structure or particle size of the substrate (e.g., solid solution(alloy), solid solutions containing crystalline inclusions (grain), andfully crystalline, as well as amorphous, glass-like etc., whilemicroscopic particles and especially nanoparticles presenting with veryspecific and often desirable properties);

condition of the surface of the substrate (e.g., polished, roughened,specifically formed and in what shape, for instance, grooved gratingsmade by deposition of alloying elements, microscopic spheres, and, inspecific cases, highly localized conditions on the surfaces ofmicroscopic particles and nanoparticles etc.);

chemical reactivity of the substrate with its environment and, in caseof analytical or preparatory surface phenomena (such as chemicalcatalysis, photonics, energy capture etc.) with the molecules of thesolvent, analyte, precursors, intermediate compounds and products ofchemical reactions, as well as desirable and undesirable trace compounds(such as atmospheric gases and water vapor) and contaminants;

stability of the substrate in regard of changes induced, inter alii, bythe incident illumination, especially with energetic photons, such asultraviolet and, in general, shorter wavelength electromagneticradiation, as well as aging, formation of fractures, dislocations andother defects;

other factors arising due to endogenic and exogenic factors, including,in one example, the size of the grain (crystalline inclusions in thealloy), as greater energy loss is known to occur on the grain boundary.

Of note is the fact that chemical reactivity of the substrate may beboth beneficial and detrimental depending on the specifics of theapplication. Also, high chemical reactivity of a specific substrate inone environment (e.g., in air or water) may change to low chemicalreactivity in a different environment (e.g, in a non-polar organicsolvent). A substrate that presents with high reactivity in water maysuffer shortened usable time due to corrosion, solubilization anddegradation; however, such substrate may also present with advantagesthat eclipse its enhanced wear and degradation.

Since the 1970s, following the initial discovery of the surfaceenhancement phenomenon, multiple scientific studies had been undertakenand published that aimed to evaluate suitability and usability ofspecific chemical elements, their alloys and mixtures for plasmonictechnologies (Arnold and Blaber, 2009).

The consensus is that development and employment of alternativematerials, rather then the commonly used coinage metals (copper, silver,and gold) or the sometimes used platinum group metals (platinum andpalladium, sometimes also ruthenium, rhodium, iridium, and osmium) arenecessary to achieve the desired enhancement of the efficacy ofplasmonic devices and technologies.

In search of such alternative materials, one needs to consider the factthat elements noted for their strong enhancement efficacy (copper,silver, and gold) are known to be in violation of the Madelung's rule ofelectron shell configuration—all three of these elements possess asingle s-electron: copper has the electron shell configuration of [Ar]3d10 4s1; silver—[Kr] 4d10 5s1, and gold—[Xe] 4f14 5d10 6s1, which mayexplain the high degree of plasmonic performance of these metals. Otherelements of interest may, therefore, include the alkali metals (lithium,sodium, potassium, rubidium and cesium) that also have an unpaireds-electron in their valence shells, especially those comparable inatomic size to the most efficacious element (silver) such as potassiumwith the electron configuration of [Ar] 4s1, rubidium—[Kr] 5s1, andcesium with the electron configuration of [Xe]6s1, although with theheavier elements relativistic effects may cause reduction of efficacy ofplasmonic performance.

Classical (Maxwellian) electrodynamics provides the following formulafor the macroscopic electronic response of a substrate:

ε(ω)=ε′+iε″

wherein ε is the permittivity of the substrate, the measure ofresistance that is encountered when forming an electric field in thesubstrate; ω is the angular frequency (pulsatance, a measure of the rateof change of the phase of a sinusoidal waveform, such as electromagneticwave); while ε′ is the real part of the permittivity, signifying thestorage of energy, in this case, on the surface of the substrate, and ε″is the complex part of the permittivity, signifying the rate of loss ofenergy due to dissipation. However, even though the formula appearssimple and straightforward, in reality, especially dealing with asurface of a substrate that is immersed in another medium (air, water,solvents, etc.) and may be also coated with layers of oxides, nitrides,other opportunistic compounds or intentionally reacted with molecules ofanalyte substance or several such substances, the resultingsuperimposition of permittivities renders calculations extraordinarilycomplicated.

Classical electrodynamics also provides the means of assessment of thequality factors that determine the efficacy of plasmonic effects for aspecific substrate when it is illuminated with photons of a narrowwavelength range, such as produced by lasers and laser diodes. Themeasure of such efficacy is dependent on the complex part of thepermittivity iε″, and is commonly assessed as the ‘Quality factor’ (QL),which has been calculated (for the narrower case of localized surfaceplasmons generated on the surface of nanoparticles, yet relatable to alltypes of plasmons) and published for the majority of chemical elementsbut not their alloys. (Blaber et al., 2010).

As expected, quality factors are high for the coinage metals(copper=10.09, silver=97.43, and gold=33.99) and for the alkali metals(lithium=28.82, sodium=35.09, potassium=40.68, rubidium=21.90 andcesium=11.20). These elements may, therefore, be suitable substrates forplasmonic technologies. The only other elements for which the calculatedQL factor appears to be appreciable are magnesium (calculated QL of9.94), palladium with QL of 6.52, and aluminum for which conflictingdata exist suggesting the QL of 13.58 or less. These data suggest thatalloys of said metals may present with desirable characteristics andhigh efficacy of their plasmonic response.

The second consideration of practical value is related to the specificsof incident illumination, which, in theory, may be of any wavelength;however, in practice, there are limitations on the range of usefulwavelengths. Photolytic effects are especially likely to occur withshorter wavelengths such as ultraviolet spectrum, destroying orsignificantly altering the substrate and, if present, molecules adsorbedon the surface of the substrate. At the same time, less energetic longerwavelength photons of the infrared and microwave spectra may be notenergetic enough to cause and sustain the development of coherentdelocalized electron oscillations. Expense and complexity are likely toproduce additional challenges in both shorter than visible and longerthan visible wavelengths.

The most economical and best studied, as well as most commonlyimplemented, are the techniques that operate with wavelengths of thevisible and near-infrared spectrum, between approximately 380 nm andapproximately 780 nm (visible) and up to 2500 nm (near-infrared). Theseimplementations work well with standard glass-based optical elements,detectors and sources, which are common and inexpensive. Visible lighthas the additional potential advantage of allowing visual monitoring ofthe substrate as well as classification of exemplars, and detection ofdefects by the human eye.

Indeed, the most common metals used in plasmonics, such as silver andgold are best for experiments utilizing visible and near-infraredspectra because their plasmon resonance frequencies fall within thatrange of frequencies. Gold, with its intrinsic yellow color, reflectswell in the 550-700 nm range (84% to 94%, a smooth curve), but absorbsfairly strongly in the shorter visible wavelengths, with the absoluteminimum at about 450 nm (35% reflectance). This makes gold a potentiallydesirable substrate for plasmonic applications with the incidentillumination in the blue-green spectrum. Similarly, the absorptionspectrum of copper also falls within the roughly similar range(Creighton and Eadon, 1991), whereas silver, platinum, and palladiumreflect all visible light relatively evenly, as signified by thesilvery-gray appearance of their surfaces and think films; however,platinum and palladium nanoscale structures, including nanoparticles andpossibly including nanoscale grooves on the surface of a solid objectmade of these metals, may also display plasmon resonance in the visibleand near-infrared wavelengths (Langhammer et al., 2006).

The wavelength that elicits the most efficacious response from thesubstrate differs depending on the chemical identity of the element.Once again, the wavelengths corresponding to maximum quality factor QLhad been approximated (Blaber M G et al., 2010) and listed below:

Barium=649 nm (visible, red)Copper=709 nm (visible, red)Gold=886 nm (near infrared)Magnesium=310 nm (near ultraviolet)Palladium=approximately 12400 nm (infrared)Platinum=3543 nm (infrared)Potassium=1182 nm (near infrared)Rubidium=1532 nm (near infrared)Silver=1088 nm (near infrared)Tin=551 nm (visible, green-yellow)Zirconium=413 nm (visible, purple)However, while barium, tin, and zirconium may present with desirableplasmonic phenomena when illuminated with visible wavelengths, theirenhancement quality factors are low, in the range of 1 to 4 units, whichrenders these elements in their pure, unalloyed state impractical fortechnological applications.

The majority of the high quality factor elements, with the exception ofsilver, gold and platinum group metals suffer with high reactivity,especially the alkali metals, which are highly reactive under standardconditions and react with air, water and many organic molecules ratherviolently. Same is true for barium, while zirconium, magnesium and,likely, tin are only passive due to the presence of a dielectric layerof a less reactive oxide, the very essential nature of which, being aninsulator, hinders plasmonics.

The search for a desirable substrate that is also sufficiently excitedby the most convenient and inexpensive type of electromagneticradiation, namely, visible light cannot, therefore, be restricted tosingle chemical elements.

However, alloys of two or several elements may possess the desirablecharacteristics: high quality factors and best or at least highefficiency of excitation in the visible spectrum. Such alloys may be inthe form of solid solutions or solid solutions with inclusions ofcrystals (grains) of intermetallic compounds.

It is empirically known that some alloys and inter-metallic compoundspresent with visible color that is different from the color of theconstituent elements. The most ancient example is electrum, an alloy ofgold and silver that possesses a fairly saturated green color. Thisphenomenon is explained by the specific alloy preferentially reflectingphotons of certain wavelengths of the visible spectrum and absorbingphotons of other wavelengths. Such absorption of photons of the visiblespectrum is a desirable characteristic, as the energy of the absorbedphotons can be used to excite surface plasmons and contribute toplasmonic phenomena (Campion and Kambhampati (1998).

SUMMARY OF INVENTION

The disclosed invention describes the compositions of improvedsubstrates for surface enhancement technologies and surface eventtechnologies. Said improved substrates are disclosed as alloys of gold,platinum and palladium with other chemical elements, especially of thealkali, such as potassium and rubidium, alkali earths, such as magnesiumand barium, as well as certain transition metals such as tin andzirconium taken in specific disclosed combinations and specificdisclosed ratios.

One desirable characteristic of substrates disclosed herein affects theelectromagnetic factor of the surface-associated phenomena, such asplasmonics. Surfaces of solid objects, thin films, and particlesmanufactured from said alloys possess different visible colors, that is,they present with desirable reflectance of specific wavelengths of thevisible light, different from the characteristic reflectances of thepure substrate metals (gold, platinum and palldaium). The change inreflectance characteristics of the alloy compared to the pure substratemetal is due to the changes in the energy of surface plasmons, as wellas the surface plasma frequencies and the different interbandtransitions in the alloy compared to the pure substrate metal. Thesechanges are not random but rather fully distinct and characteristic andare due to the influence of the alloying chemical elements and formationof specific phases and crystal structures in the alloy. The use of saidsubstrates, therefore, allows for optimization of resonance at aparticular wavelength of the incident light, a highly sought aftercharacteristic in all varieties of plasmonics applications.

The second desirable characteristic of disclosed substrates affects thechemical factor of surface phenomena. While pure substrate metals,especially gold and platinum, and, to the lesser extent, palladium andsilver, are fairly inert (‘noble’) and have lesser affinities for manymolecules and functional groups, the alloys as disclosed are potentiallymore reactive, allowing to stronger coupling and different types ofcoupling, thus causing a change in the adsorbed molecule, its shape,conformation, density of its electron cloud, position and shape offunctional groups and other characteristics. Therefore, the chemicalenhancement variation due to the presence of alloying elements withintrinsically different electrochemical properties, different reactivityand affinity for various functional groups of the adsorbed molecules isuseful in a variety of plasmonics technologies.

Embodiments of the present invention include alloys of gold, platinumand palladium presenting with distinct and uniform values of surfaceplasmon generation as well as surface reactivity that are different fromthe constituent elements.

The disclosed substrates may be used as bulk and macroscopic objects,thin films, and particles on the micrometer and nanometer scales.

The present invention posits the use of specifically formulated alloysof said metals to facilitate the following:

changes in resonance frequencies of the surface plasmons, allowingexcitation with a wider range of photon wavelengths or, conversely,allowing excitation only with specific ranges of wavelengths, thus,potentially reducing the interfering effects of fluorescence;

formation of chemical bonds between the adsorbed molecule and thecomponents of the alloy, such as separate atoms and crystallineinclusions (grains);

utility in photovoltaic, that is, conversion of photic energy toelectric potentials or currents; photocatalytic, that is facilitation ofchemical reactions utilizing the energy of electromagnetic radiation;plasmon-based electronics, that is, utilizing different alloys asdisclosed to form dual- and triple-layered structures that can functionas diodes and transistors.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 presents the graphical representation of measurement ofreflectance of three experimental alloys (2Au:1K, 2Au:1Rb, and 5Au:1Rb)as well as pure gold (Au).

FIG. 2 presents the graphical representation of measurement ofreflectance of two experimental alloys (1Mg:1Pt:1Sn and 1Mg:1Pd:1Sn) aswell as pure gold (Au).

DESCRIPTION OF EMBODIMENTS

The inventors, a professional jeweler and a researcher in the field ofnon-linear optics, embarked upon creation of alloys of gold, platinumand platinum group metals suitable for plasmonic applications with theunderstanding of the following criteria:

criterion A. The alloy needs to be visibly colored, preferentiallyabsorbing and reflecting certain parts of the visible spectrum;criterion B. The alloy needs to be prepared in a facile fashion.

In the experiments carried out and directed solely by the inventors,series of alloys of various composition were prepared both byconventional techniques (alloying under inert gas) and by alternativetechniques (vapor deposition, sintering, electrochemical deposition frompolar and organic electrolytes). Reflectance of obtained alloys wasmeasured under standard conditions with a scanning reflectometer. insome of the cases, such as alloys with high reactivity andsusceptibility to tarnishing in air, reflectances were measured in thesame quartz ampoule in which alloys were prepared.

The alloys were prepared from chemically pure (no less than 99.9%purity, under vacuum) elements obtained from Smart Elements GmbH(Austria) and from a Russian commercial supplier. Gold, palladium andplatinum were processed by a bullion supplier (Produits ArtistiquesMétaux Précieux, Ticino, Switzerland), all sealed in original cards andassayed with 99.99% or better marks of purity. Due to the high goldcontent of the alloys and the need for multiple samples, 99% pure goldwas also prepared from jewelry scrap by a modified electrolyticdissolution method and alloys prepared from such lesser purity gold wereused in preliminary experiments.

Preparation of alloys presented with some challenges, especially alloysmade from metals with vastly different melting points (e.g., gold andrubidium). In those cases, the more difficult to melt metal was firstrolled into thin foil (˜0.05 mm thick for gold, ˜0.08 mm thick forplatinum and palladium), weighted, acid washed and ultrasonicallycleaned, dried, re-weighted, and introduced into quartz ampoule with theother metal already present under protective atmosphere of argon gas.Metals supplied under protective mineral oil (such as alkali metals)were washed in a great excess of n-hexane under argon atmosphere and theresidual n-hexane was evaporated with a stream of argon gas at roomtemperature. The ampoule was introduced into an electric furnace filledwith argon gas and heated with periodic agitation. Temperatures for thealloy formation were taken from multiple references and corresponded tobinary alloy phase diagrams. In all cases the samples were overheated by˜50 degrees C. to allow for liquidus formation. Cooling of the preparedsamples was at the rate of 10 degrees C. per hour around the expectedsolidus formation temperature, and subsequently at 100 degree C. perhour, allowing for formation of a suitably looking sample. Thepreparation of the gold-zirconium alloy was carried under similarprocedure, except zirconium foil was introduced into a quantity ofmolten gold and incubated for 24 hours at 1280 degrees C.

The technical difficulty of preparation of alloys of gold with reactivemetals, such as alkali metals, prompted the development of a principallynovel method of manufacturing of alloys, especially suitable forpreparation of alloys of metals with vastly different reactivity, suchas alloys of gold and alkali metals.

This method consisted of the following steps:

foil of the main element of the alloy such as gold, platinum orpalladium was produced by conventional metallurgy methods such as coldrolling, resulting in a foil with minimal surface roughness as evidentby a mirror-like sheen;

said foil was thoroughly cleaned and immersed into water-free non-polarorganic electrolyte comprised of one or several of the followingchemical compounds: propylene carbonate (CH3C2H3O2CO), ethylenecarbonate ((CH2O)2CO), dimethyl carbonate (OC(OCH3)2), poly(oxyethylene)(a polymer with the general formula HO—(CH2CH2O)n-H), diethyl carbonate(OC(OCH2CH3)2), and similar organic electrolytes, or, in some of theexperiments, into ionic liquids such as ethylammonium nitrate also knownas ethylamine nitrate (C2H8N2O3) and similar compounds in which wasdissolved a salt of the alloying element, such as potassium chloride(KCl), rubidium chloride (RbCl) or cesium chloride (CsCl) with optionaladditive in the form of aluminum chloride (AlCl3);

electric current was introduced with the foil serving as cathode and agraphite rod serving as the anode;

electrochemical deposition of the alloying element such as potassium,rubidium, cesium on the surface of the foil made of the main element ofthe alloy occurred and the amount of the deposited alloying element wasquantified by determination of voltage and current;

the resulting foil with electrochemically deposited alloying metal wastaken out of the electrolyte, washed with n-hexane and dried under astream of argon;

subsequent treatment included heating under protective argon atmospherefor 24 hours at 500 degrees C., facilitating diffusion of atoms andformation of the alloy, or, optionally, heated to higher or lowertemperature depending on the identity of the alloying element (not toexceed the boiling point of potassium at 759 degrees C., boiling pointof rubidium at 688 degrees C., or boiling point of cesium at 671 degreesC., correspondingly);

mechanical treatment of the resulting alloy followed and includedsmoothing, rolling or burnishing with an agate rod under the protectiveatmosphere of argon gas.

Upon examination, surfaces of thus manufactured alloys were found topresent with specific colors, such as the dark green color when theamount of electrochemically deposited rubidium was 17.82 weight percentto 82.17 weight percent of gold.

A variation of the aforementioned method comprised a preparatory step,in which the goal was to produce a roughened surface of the foil of themain element of the alloy, such as gold prior to the electrochemicaldeposition of the alloying element. The usefulness of such roughenedsurface is well known in plasmonics and the resulting nano-scale (<100nm) striated and sponge-like surface features upon electrochemicaldeposition of the alloying element will form a striated or sponge-likesurface of the alloy.

To achieve the roughening of the surface of the foil made from the mainelement of the alloy and formation of nanoscale surface features, anintermediary alloy of the main element was created. In one approach, theintermediary alloy was comprised of less than 25 weight percent of goldwith the balance being silver. Specifically, the intermediary alloy wasprepared by co-melting under argon atmosphere of 22 weight percent puregold no less than 99.99 purity and 78 weight percent pure silver of noless than 99.9 purity. The resulting alloy was cold rolled to 1 mmthickness and subjected to acid etching with aqueous nitric acid(specific gravity of 1.25 g/ml) for several hours at ˜95 degrees C. (thehot acid etching was chosen as cold acid etching resulted in generationof fine particles) with subsequent washes in boiling water, followedwith a prolonged wash in 1 percent solution of urea in water toneutralize remaining residues of nitric acid and final washes withboiling and cold water.

The resulting foil of pure gold was observed under the microscope tohave sponge-like surface and was used in the aforementionedelectrochemical deposition process resulting in a formation of asponge-like structure of the alloy of the alkali metal with gold.

Alternatively, to generate a surface characterized by nanoscalestriations, a quantity of pure gold foil was pressed with a quantity ofpure copper foil pre-washed with boiling 10% solution of citric acidwith subsequent boiling water washes under protective atmosphere toremove oxidation layers from the copper foil. With application ofconstant pressure at 100 kg/cm2 the bilayer foil was incubated at 400degrees C., facilitating the formation of Au—Cu intermetallics. Thefused bilayer foil was then subjected to boiling in nitric acid untilfull dissolution of the copper component. The surface of the remaininggold component was found to be striated with the average size of thestriation less than 100 nm. This gold foil with striated surface wassubsequently subjected to electrochemical deposition of the alkali metalas described above.

Other intermediary alloys, such as a ternary alloy of gold-silver-copperand quartenary alloy of gold-silver-copper-zinc were also successfullyused to generate other types of roughened surfaces.

Other etching solutions were used, including various concentrations ofnitric acid-water mixtures, other inorganic acids, including selenicacid (H2SeO4), water-based solutions that generated halogens in situ,such as the mixture of sodium hypochlorite (NaClO) with hydrogenperoxide (H2O2), solutions of free halogens, such as iodine-iodidesolution (I2 in KI), as well as salts of the thiosulfate anion (Na2S2O3)under acidic pH and solutions of thiourea under various pH. All thesesolutions were utilized upon the primary dealloying of the intermediatealloy with the goal of etching the gold component with either smoothingof the surface or, alternately, increased roughening of the surface.

The aforementioned methods were utilized to produce multiple samples ofgold-based, as well as platinum- and palladium-based alloys withdesirable characteristics, such as desirable surface color and desirablereflectance curves. Other methods for alloy preparation are well knownto persons familiar with the metallurgic science and are not a part ofthe disclosed invention.

Multiple samples of the gold alloys were prepared, with alloyingelements added in weight percentages that fell into the range indicatedin Table 1, for example, for the Gold-Potassium alloy 1 four sampleswere prepared: with potassium at 2.5, 3.0, 3.5, 3.82 (calculated idealratio corresponding to the 1/6 atomic parts of K to 5/6 atomic parts ofAu) and 4.0 weight percent. The samples were evaluated for desirablecharacteristics, such as change in reflectivity spectrum compared to thepure metals (gold, platinum and palladium) and reactivity with air,water and common solvents including anhydrous ethanol, acetone andn-hexane.

Composition of alloys, especially the atomic percentages of thecomprising chemical elements, was confirmed with X-ray fluorescenceutilizing the Bruker S1 Titan XRF analyzer.

Of the large number of prepared alloys, only a small portion was foundto exhibit the desired characteristics as discussed above and presentedbelow:

Atomic Weight Formation Color as Alloyed elements Label ratios percentat ° C. observed Gold, Potassium 5Au: Au: 5/6; Au 96.18%, 800-900 darkgreen 1K K: 1/6 K 3.82% Gold, Potassium 2Au: Au: 2/3; Au 90.97%, 650reddish- 1K K: 1/3 K 9.03% purple Gold, Rubidium 5Au: Au: 5/6; Au 92.2%,730 dark green- 1Rb Rb: 1/6 Rb 7.98% yellow Gold, Rubidium 2Au: Au: 2/3;Au 82.17%, 580 dark green 1Rb Rb: 1/3 Rb 17.82% Gold, Barium 5Au: Au:5/6; Au 87.76%, 800-900 gray-blue, 1Ba Ba: 1/6 Ba 12.24% shimmeringGold, Zirconium 3Au: Au: 3/4; Au 86.62%, 1280  gray-blue, 1Zr Zr: 1/4 Zr13.37% shimmering Gold, Magnesium, 1Mg: Au: 1/3; Au 57.93% 780 reddish-Tin 1Au: Mg: 1/3; Mg 7.15%, purple 1Sn Sn: 1/3 Sn 34.92% Platinum, 1Mg:Pt: 1/3: Pt 57.70% 920 dark Magnesium, Tin 1Pt: Mg: 1/3; Mg 7.19%,reddish- 1Sn Sn: 1/3 Sn 35.11% orange Palladium, 1Mg: Pd: 1/3; Pd 42.55%880 brass Magnesium, Tin 1Pd: Mg: 1/3; Mg 9.75%, yellow 1Sn Sn: 1/3 Sn47.59%

Six of the listed alloys exhibiting color when viewed with the naked eyewere subjected to measurement of reflectance of the visible light. Theresults of these measurements are presented in FIG. 1 and FIG. 2.

The measurements were performed at room temperature and normalbarometric pressure in the visual spectrum extending from 400 nm(violet) to 700 nm (deep red) utilizing a custom made spectroscopicreflectometer set at 20 degree incident angle reporting totalreflectance at 7 bands set at 400, 450, 500, 550, 600, 650 and 700 nmwhich were supplied by a tunable source of near-monochromatic light(Fastie-Ebert in-line monochromator, effective aperture of f/4 and focallength of 75 mm with 1800 lines/mm diffraction grating and 0.5 mm slitwidth, with wavelength precision of approximately 7.5 nm). Reflectance(R) was measured as:

R=ϕr/ϕi

Where ϕr is the radiance reflected by the surface and ϕi is the radiancereceived by the surface.

FIG. 1 presents a plot of experimental data obtained by measuringreflectance of four samples: 99.99% pure gold (reference sign 10); alloy2Au:1K (reference sign 21), appearing reddish-purple to the eye; alloy2Au:1Rb (reference sign 22), appearing dark green to the eye; alloy5Au:1Rb (reference sign 23) appearing dark greenish-yellow to the eye.

It is evident from FIG. 1 that all three alloys of gold with alkalimetals possessed significantly different reflectance characteristics,representative of desirable changes in the absorption and re-emittanceof incident visible light that correspond to desirable parameters ofsurface plasmon polaritons.

FIG. 2 presents a plot of experimental data obtained by measuringreflectance of two experimental samples overlaid with the data from FIG.1 for pure gold (reference sign 10) for comparison. The two experimentalalloys were 1Mg:1Pt:1Sn (reference sign 31), appearing darkreddish-orange to the eye and 1Mg:1Pd:1Sn (reference sign 32) appearingbrass-yellow to the eye.

Measurements were performed at same wavelengths as for FIG. 1 andcalculation of reflectance was also performed as for FIG. 1. From themeasured data it is evident that both alloys, of magnesium and tin withplatinum or palladium also possessed significantly different reflectancecharacteristics, representative of desirable changes in the absorptionand re-emittance of incident visible light that correspond to desirableparameters of surface plasmons.

Of these, as expected, the Gold-Potassium, Gold-Rubidium and Gold-Bariumalloys, despite the very high weight percent of gold in the composition,exhibited higher reactivity and tarnished under humid air, necessitatingmeasurement of reflection while the sample was confined in a quartzampoule and its exposed surfaces were coated with mineral oil. TheGold-Zirconium, Gold-Magnesium-Tin, Platinum-Magnesium-Tin andPalladium-Magnesium-Tin alloys were less reactive, however, in someinstances they were also measured while confined to a quartz ampoule andtheir exposed surfaces coated with mineral oil of same composition asthe alkali-gold alloys.

Alloys presented in Table 1 were exhibiting significant visible colorthat was distinct from the colors of the constituent metals and were,therefore, considered especially desirable, as they met the abovementioned Criterion A. The purple-reddish colored alloys (such asGold-Potassium 2Au:1K and Gold-Magnesium-Tin alloy) presented with verystrong interband electronic transitions around 3 eV, corresponding tophotons of wavelengths around 410-420 nm, explaining their visible colorand the reflectance curve; while the electronic transitions of thegreen-colored alloys are likely to correspond to the energies of2.0-2.75 eV (wavelengths of approximately 450 nm and 600 nm). Sincethese wavelengths are in the visible spectrum, the Criterion A is metundeniably; while the electrolytic deposition method of manufacturingallows these alloys to meet Criterion B.

Of note is the opportunity to produce gold and platinum groupmetal-based alloys of alkali or alkali-earth metals formed into objectsor particles which are then partially dealloyed with water orwater-organic solvent mixtures. Roughness of the surface is achieved byreaction with water in large or trace amounts and surface structuresthat are cubic-derived and hexagonal derived are formed, providing anentirely different type of roughening of the surface compared with theconventional acid etching of pure gold and silver. If the dealloying isstopped prior to complete removal of the reactive atoms, the remainingatoms of the alloying element (such as rubidium in gold-rubidium alloysand magnesium in palladium-magnesium-tin alloys) may serve as partialenhancers, especially of the chemical enhancement. Thepalladium-magnesium-tin alloy undergoes partial deallying upon etchingwith weak organic acids and may serve as a replacement of the moreexpensive solid palladium in a multitude of applications. The dealloyedgrainy structure rolled or beaten with a cover gold leaf as thin as toallow green-blue pass through as well as formation of evanescent wavesmay also be used.

Other technical advantages and applications of the disclosed inventionmay become readily apparent to one of ordinary skill in the art uponfamiliarization with the disclosed figures and description. The scope ofthe invention is defined by the scope of the claims.

We claim:
 1. A composition of matter such as an alloy with the properties of: (i) being comprised of at least one metal selected from gold (Au), platinum (Pt), and palladium (Pd) as the main element of the alloy; (ii) being comprised of at least one alloying component promoting change in the energy of surface plasmons of the resulting alloy preferably chosen from alkali metals such as potassium (K), and rubidium (Rb), as well as alkali earth metals such as magnesium (Mg), and barium (Ba), as well as transition metals inclusive of tin (Sn), and zirconium (Zr); (iii) the resulting alloy necessarily and essentially presenting with the property of substantially different energy characteristics of the surface plasmons as present on the surface of objects made from the alloy, including solid items, thin layers and nanoparticles, as compared to any or all of the constituent elements and assessed by one or all of the following manifestations: a change of surface color as perceived by the human eye as a function of reflectance of specific wavelengths of incident white light; a change in specific absorbance and reflectance characteristics when illuminated with electromagnetic radiation including ultraviolet, visible, and infrared light as polychromatic mixture of wavelengths, monochromatic illumination or illumination with multiple narrow wavelengths; a change in efficacy of generation, surface plasmon resonance frequency, specific energies of surface plasmons generated upon illumination of the surface of the alloy with electromagnetic radiation, including ultraviolet, visible and infrared light as polychromatic mixture of photons of multiple wavelengths, monochromatic illumination or illumination with photons of multiple narrow ranges of wavelengths.
 2. A composition of matter comprising: an alloy consisting primarily of gold and potassium at atomic ratios of two parts of gold to one part of potassium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as reddish-purple as perceived by the human eye, with said color development being of essence.
 3. A composition of matter comprising: an alloy consisting primarily of gold and potassium at atomic ratios of five parts of gold to one part of potassium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as dark green as perceived by the human eye, with said color development being of essence.
 4. A composition of matter comprising: an alloy consisting primarily of gold and rubidium at atomic ratios of five parts of gold to one part of rubidium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as dark green-yellow as perceived by eye, with said color development being of essence.
 5. A composition of matter comprising: an alloy consisting primarily of gold and rubidium at atomic ratios of two parts of gold to one part of rubidium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as dark green as perceived by the human eye, with said color development being of essence.
 6. A composition of matter comprising: an alloy consisting primarily of gold and barium at atomic ratios of five parts of gold to one part of barium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as gray-blue and shimmering as perceived by the human eye, with said color development being of essence.
 7. A composition of matter comprising: an alloy consisting primarily of gold and zirconium at atomic ratios of three parts of gold to one part of zirconium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as gray-blue and shimmering as perceived by the human eye, with said color development being of essence.
 8. A composition of matter comprising: an alloy consisting primarily of gold, magnesium and tin at atomic ratios of one part of gold to one part of magnesium and one part of tin, with deviation from said ratios not exceeding two atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as reddish-purple as perceived by the human eye, with said color development being of essence.
 9. A composition of matter comprising: an alloy consisting primarily of platinum, magnesium and tin at atomic ratios of one part of platinum to one part of magnesium and one part of tin, with deviation from said ratios not exceeding two atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as dark reddish-orange as perceived by the human eye, with said color development being of essence.
 10. A composition of matter comprising: an alloy consisting primarily of palladium, magnesium and tin at atomic ratios of one part of palladium with one part of magnesium and one part of tin, with deviation from said ratios not exceeding two atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as brass-like yellow as perceived by the human eye, with said color development being of essence. 